The present invention relates to a direct gas sensor and more particularly to a hydrogen sulfide direct gas sensor.
Hydrogen sulfide (H.sub.2 S) is an extremely toxic gas and exposure to concentrations of less than 0.1% in air for a few seconds can be fatal. The Occupational Safety and Health Act (OSHA) has stipulated that 10 parts per million (10 p.p.m.) is the maximum weighted-averaged limit that a person can be continuously exposed to in an 8-hour working day.
Hydrogen sulfide is found in many industrial environments, but is particularly prevalent in the petrochemical industry. It is necessary in such industries to have an accurate, low-cost, low maintenance H.sub.2 S monitor which not only has a fast response time, on the order of less than 5 seconds, but also has a fast recovery time. Present day sensors are mostly solid state and are not particularly selective. For example, some such solid state sensors respond to ammonia. They typically have a slow response and if they have not been in contact with H.sub.2 S for a day or two they can take up to four minutes to respond to the OSHA limit. Most H.sub.2 S sensors require explosion proofing and must not be corrodible. Many types of prior art H.sub.2 S sensors, besides being slow and cumbersome, also have to be checked out every day, particularly those sensors which use electrochemical methods. Furthermore, many such electrochemical type sensors are mechanically complex, requiring pumps, pressure monitors and the like.
One particular type of sensor electrode is known in the art as a specific ion probe. Typically, such specific ion probes utilize a specific ion electrode which is sensitive to the specific ion which is to be sensed, such as S.sup.=. A specific ion electrode is utilized in conjunction with a reference electrode which may take a variety of forms. The reference electrode and the specific ion electrode are immersed in a solution in which the gas containing the specific ion is bubbled. The electrochemical reactions at the two electrodes produce a voltage in proportion to the logarithm of the concentration of the specific ion in the solution. The primary problem with such arrangements is that they generally require the probes to be immersed in the solution. These probes are also not particularly fast when the time required to introduce the gas into the solution is taken into account.